1. Field of the Invention
The present invention relates to a sheet conveying apparatus that conveys sheet-like members such as a recording sheet and a sheet-like original, and a belt drive apparatus that endlessly moves an endless belt member. The present invention also relates to an image reading apparatus and an image forming apparatus that use the sheet conveying apparatus or the belt drive apparatus.
2. Description of the Related Art
As an image forming apparatus of this type, an image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2005-41623 is known. The image forming apparatus forms an image on a printing paper by a known electrophotographic process, while feeding the printing paper in a paper feed tray to a paper feed path, and conveying the printing paper. An optical displacement sensor that detects the displacement of the printing paper fed out to the paper feed path is provided in the paper feed tray. The optical displacement sensor is widely used in an optical mouse and the like, that is an input device for a personal computer. The optical displacement sensor optically detects two-dimensional displacement of a printing paper, which is an object to be detected. When a paper feed roller that feeds a printing paper from the paper feed tray and a pair of conveying rollers that applies conveying force to a printing paper in the paper feed path are deteriorated, a so-called skew in which the printing paper is not conveyed in an upright position along the conveying direction, but conveyed in a tilted position starts to occur. By detecting the moving distance of the printing paper in the direction perpendicular to the conveying direction caused by the skew with the optical displacement sensor, the life expectancy of the paper feed roller and the pair of conveying rollers can be predicted, whereby the user is prompted to replace the rollers before the rollers are broken.
The optical displacement sensor includes a light emitting element that emits light to the object to be detected, and a plurality of light receiving elements that receives reflection light obtained on the surface of the object to be detected. The light receiving elements, for example, are arranged in a matrix, as shown by reference numerals 900 in FIG. 1. Fine irregularities are present on the surface of the object to be detected. Accordingly, regions where the amount of reflection light is significantly high, and regions where the amount of reflection light is significantly low (hereinafter, the regions are referred to as “characteristic location”) are present on the surface of the object to be detected. The optical displacement sensor obtains two-dimensional displacement at the characteristic location, based on the time series variation of the amount of received light, of the light receiving elements arranged in a matrix. For example, the position of a light receiving element 900 that receives more reflection light varies in time series, as shown in arrows in FIG. 1, with the movement of the characteristic location of the object to be detected. Accordingly, two-dimensional displacement along the arrows of the characteristic location can be obtained.
When the present inventors carried out an experiment to detect the amount of skew of a sheet, by mounting a commercially available optical displacement sensor on a paper feed path of a printer testing machine, the inventors have found out that sensitive detection is difficult. More specifically, nearly all the commercially available optical displacement sensors are developed for optical mice. Accordingly, two-dimensional displacement can only be identified in a very narrow area of the surface of the object to be detected. For example, in the example shown in FIG. 1, a detection area is a sheet area corresponding to a matrix of 16×16 pieces of the light receiving elements 900. However, this is just a very small area of the entire area of the sheet. In FIG. 1, if the direction of the arrow Y is the conveying direction of the sheet, the characteristic location of the sheet moves the entire area in the direction of the arrow Y without fail. At this time, if the sheet is skewed, the sheet also moves in the direction of the arrow X, as well as in the direction of the arrow Y. However, the moving distance in the direction of the arrow X is only a small amount, compared to the moving distance in the direction of the arrow Y. Accordingly, even if the characteristic location of the skewed sheet is moved in the direction of the arrow X in the narrow detection area in FIG. 1, the moving distance is as much as a few pixels (few pieces of light receiving elements). In the example in FIG. 1, such a small amount of moving distance of a few pixels in the direction of the arrow X can only be obtained by a unit of pixel, such as one pixel and two pixels. Consequently, it is difficult to sensitively detect the amount of skew.
The problem when the amount of skew of the sheet in the conveying path such as the paper feed path of the image forming apparatus has been described. However, the similar problem occurs, when the amount of skew of an original in a conveying path of an automatic document feeding device of a scanner is to be detected. The similar problem also occurs, when a configuration in which an optical displacement sensor detects the bias amount of a belt member in the width direction is adopted, in a belt drive apparatus that endlessly moves an endless belt member such as an intermediate transfer belt.